1. Field of the Invention
The present invention is related generally to heat flux transducers and, more particularly, to a differential thermopile heat flux transducer having a plurality of layers and a high thermal conductivity top coating which enables the transducer to measure heat flux with improved accuracy.
2. Description of the Related Art
Heat flux sensors are routinely used to measure the rate and direction of heat energy flow. For example, heat flux sensors (or "transducers") have been used in building energy management applications since the 1950's. Methods for using heat flux transducers to evaluate thermal performance of building materials are generally well understood. Heat flux transducers designed for surface mounting are inexpensive and easy to install. However, it is often difficult to acquire accurate, useful data with heat flux sensors.
In combination with a temperature sensor disposed at the same location, a heat flux sensor and a temperature sensor can be used to measure temperature, heat flux, the heat transfer coefficient, the effective thermal capacity, the projected rate of temperature change at the present heat flux, and the projected rate of temperature change at any other value of heat flux. In addition, the temperature and heat flux signals can be compared to detect drift or failure of either sensor.
Measurement of heat flux is critical to the understanding and control of many thermal systems. When both heat flux and temperature data are available for the same point on a surface, material properties such as thermal resistance and thermal diffusivity can be calculated. Heat flux measurement is essential for the performance evaluation of insulative building materials. This is because it is often difficult or impossible to predict the installed performance of insulative materials from laboratory experiments.
The heat flux through a surface cannot, however, be measured without some disturbance caused by insertion of the measuring device into the path of heat flow. The amount of change produced by the measuring device depends on many factors. These include: the contact resistance between the heat flux transducer and the test wall, as well as other physical parameters such as surface emissivity, surface roughness and the thermal resistance of the heat flux transducer itself. These factors include the effective series thermal resistance, R.sub.m. The following relationship is described in Trethowen, H. A., "Systematic Errors with Surface-Mounted Heat Flux Transducers and How to Live with Them", In-Situ Heat Flux Measurements in Buildings--Applications and Interpretations of Results, CRREL Special Report 91-3, 1991, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, N.H., Hanover, N.H., pp. 15-27: EQU R.sub.m =R.sub.h +R.sub.c +(R.sub.ms -R.sub.s) (1)
Where:
R.sub.m =effective series thermal resistance PA1 R.sub.h =series thermal resistance (conductive) of the heat flux transducer alone PA1 R.sub.c =thermal contact resistance between the heat flux transducer and substrate PA1 R.sub.ms =total thermal surface resistance (convective and radiative) over the heat flux transducer PA1 R.sub.s =total thermal resistance (convective and radiative) over surrounding area
The effective series resistance of a surface mounted heat flux transducer is the most important single factor affecting the error produced by disturbance of the measured heat flux. If R.sub.ms and R.sub.s are made approximately equal by matching the emissivity and the surface roughness of the heat flux transducer to corresponding values for the surrounding material, the effective series resistance is reduced to the sum of the heat flux transducer series thermal resistance and the thermal contact resistance. The series thermal resistance can vary widely in commercially available surface mounted heat flux transducer's: from about 0.002 m.sup.2.degree. C./W to 0.1 m.sup.2.degree.C./W. The thermal contact resistance is minimized by attaching the sensor to the surface with a very thin layer of high thermal conductivity adhesive.
For maximum utility in building energy management, a heat flux sensor should have a series thermal resistance of less than 1.times.10.sup.-4 m.sup.2.degree. C./W. However, when the series thermal resistance of a thermopile type heat flux transducer is very low, its sensitivity may also be low because the low thermal resistance only produces a small temperature difference.
Another factor which affects the utility of a heat flux sensor in building energy management is shunting of heat flux around the sensor. A common way of solving this problem is to use a sensor with a large area, in the belief that the long path around the sensor will reduce the effects of shunting. This solution is not effective when the sensor has high thermal resistance, because the measured heat flux is that which passes through the sensor, and it is reduced by the sensor's thermal resistance. A more effective solution is to employ a sensor with low thermal resistance.
Large area sensors are also commonly used to measure heat flux over non-homogeneous areas, such as across wall studs in framed buildings, or on truss roofs. Unless the areas of the sensor covering the wall studs and the surrounding structure are in the same proportion as in the entire structure, this practice introduces an error. A better way of measuring heat flux over non-homogeneous areas is to employ small heat flux sensors over each representative part of the structure, and then calculate the total heat flux for each part using its actual total area. The total heat flux is then calculated by summing these amounts.
Another disadvantage of large area heat flux sensors is that they are relatively expensive. According to a recent survey, the typical cost of a commercially available 12" by 12" heat flux sensor is over $600.
Copper conductors of printed circuit boards may be produced by a number of processes. The most common of these is photoetching. In this process, a board completely coated with copper and covered by a photopolymer is (1) exposed to ultraviolet light through a negative transparency of the desired conductor pattern, (2) solvent washed to remove the polymer where it has not been hardened by exposure and (3) acid etched to expose the desired conductors.
A second process for producing printed circuit boards, known as the additive process, consists of a first step of electroless deposition of a very thin nickel layer representing the desired conductor pattern, followed by a second step of electrolytic deposition of the desired thickness of copper on the nickel conductors.
In a third process, which is less frequently used, conductors are deposited as inks on an insulating substrate by screen printing. The ink traces are dried to a solid by rapid heating in a vapor reflow oven, then converted to metal by an elevated heat treatment. This process could be adapted to heat flux sensor manufacturing, if it could be used to deposit conductors of two different metals in an appropriate pattern.
In U.S. Pat. No. 4,779,994, a thin film heat flux sensor and its method of manufacture are disclosed. The manufacturing method has certain drawbacks which limit the sensor performance and restrict the range of applications. The manufacturing cost of the sensor is high because it is made by multiple stages of sputtering through shadow masks. Another drawback is the relatively low sensitivity of the sensor. In U.S. Pat. No. 4,779,994, heat flux is measured by measuring the temperature drop across a very small thermal resistance, and signals are of the order of a few microvolts per watt/cm.sup.2. In many applications for such sensors, heat flows to be measured are a small fraction of 1 watt/cm.sup.2, and thin film sensors cannot be used.
Prior heat flux sensors also suffer additional drawbacks. Prior heat flux sensors fail to be equally responsive in sensing radiation, convection and conduction heat transfer. Moreover, such prior heat flux sensors are not responsive to heat transfer in both directions through the sensor. In addition, such prior sensors cannot be bent over a radius without loss of continuity or calibration.